Wheel driving through objects until they reach the wheel centre?

I'm creating a monster truck type vehicle using wheel colliders but a problem I'm having is when driving over objects the wheels go through the object. Only when the centre of the wheel touches the object it starts driving over it. I've tried a capsule, box and mesh collider but it's the same with all of them.

If this is normal behaviour for wheel colliders how should it be addressed? Or have I messed up the wheel settings?